As shown in FIG. 1, a nuclear power station conventionally includes a reactor pressure vessel 10 with various configurations of fuel and reactor internals for producing nuclear power. For example, vessel 10 may include a core shroud 30 surrounding a nuclear fuel core 35 that houses fuel structures, such as fuel assemblies 40. Core 35 may be bounded vertically by top guide 45 and core plate 70. Fuel assemblies 40 may extend between and seat into core plate 70 and top guide 45, which may include several openings shaped to receive ends of assemblies 40. Other core structures, such as control elements and instrumentation tubes, may likewise extend through and/or between core plate 70 and/or top guide 45. One or more control rod drives 1 may be positioned below vessel 10 and connect to control rod blades or other control elements that extend among fuel assemblies 40 within core 35.
An annular downcomer region 25 may be formed between core shroud 30 and vessel 10, through which fluid coolant and moderator flows into the core lower plenum 55. For example, in US Light Water Reactor types, the fluid may be purified water, while in natural uranium type reactors, the fluid may be purified heavy water. In gas-cooled reactors, the fluid coolant may be a gas such as helium, with moderation provided by other structures. The fluid may flow upward from core lower plenum 55 through core 35. In a boiling water-based reactor, a mixture of water and steam exits nuclear fuel core 35 and enters core upper plenum 60 under shroud head 65.
Nuclear reactors are refueled periodically with new fuel to support power operations throughout an operating cycle. During shutdown for refueling, the vessel 10 is cooled, depressurized, and opened by removing upper head 95 at flange 90. With access to the reactor internals, equipment may be shifted or removed and some or all of fuel bundle assemblies 40 may be replaced and/or moved within core 35. Maintenance on other internal and external structures may be performed during such an outage.
As shown in FIGS. 2A and 2B, one or more fuel support castings 48 may sit on and/or extend through core plate 70. Casting 48 may include several orifices 49 to receive fuel assemblies and/or control elements, aligning them with respect to one another and with core plate 70 and directing coolant up through such components. Casting 48 may accommodate several fuel assemblies in various orifices 49 while maintaining other space on core plate 70. For example, an instrumentation tube 50 may penetrate core plate 70 and be positioned next to casting 48, allowing tube 50 to extend vertically adjacent to several fuel assemblies positioned in casting 48.
Similarly, one or more source holder penetrations 75 may extend into core plate 70 adjacent to casting 48. Source holder penetration 75 may hold a startup source, such as a sealed Californium or Plutonium-Beryllium isotope that emits substantial and detectable neutron spectra, which reliably begins the nuclear chain reaction in a new core with completely fresh fuel, or after excessively long shut-down periods when spontaneous fission is unreliable in burnt fuel. Co-owned “General Electric Systems Technology Manual,” Dec. 14, 2014, Chapter 5.1, describes helpful technological context and is incorporated by reference herein in its entirety. As seen in the top-down view of FIG. 2B, source holder penetration 75 may position the source in a desired static relation with instrumentation tube 50, permitting detection of neutrons from a source in penetration 75 to compare to neutrons generated through fission during startup, and fuel assemblies in casting 48. In this way, core plate 70 and casting 48 may radially/horizontally align several different core components at a base of a core and ensure they maintain desired positioning throughout an axial/vertical extent of the core.